Page 339 - Tandem Techniques

Page 323

Consequently, some restrictions are placed on the choice of phase systems that are required to effect the
separation.

Interface for the Combination of Liquid Chromatography and Raman Spectroscopy

Tandem systems involving the combination of the liquid chromatograph with the Raman spectrometer
are not widespread, largely due to inherent insensitivity of the earlier instruments. However, the use of
the laser as the excitation energy source has significantly increased the sensitivity of Raman
measurements. Consequently the LC/Raman combination is likely to become as practical and useful as
the LC/FTIR tandem system. The advantages of Raman spectroscopy over infrared spectroscopy lies in
the fact that Raman spectra can be readily obtained in aqueous media. This renders the LC/Raman
system particularly attractive to the biotechnology field. The structural information provided by the
Raman spectrum is very similar to that provided by the IR spectrum. The Raman spectrometer has been
connected to the liquid chromatograph to provide Raman spectra of eluted solutes in real time by Sheng
et al. [24]. In order to obtain the necessary sensitivity in a flow through cell, the authors employed the
technique of surface enhanced Raman spectroscopy (SERS). They mixed the column eluent with a
suspension of fine particles of silver which acted as the enhancing surface. The use of the technique
depends critically on the operating conditions, and particularly on the physical properties of fluid
entering the cell and the surface characteristics size, and degree of aggregation of the silver particles.
The general arrangement of their basic system is depicted in Figure 8.25.

A standard liquid chromatograph is used and the outlet is fed to a mixing T. The silver solution is
pumped through a heat exchanger to the opposite limb of the mixing T, and the mixture is taken from
the center limb. To realize maximum mixing, the outside limbs of the mixing T are arranged to be at an
angle of 30° to the center limb [25]. The conduit between the T and Raman cell must have a minimum
length to ensure adequate mixing. However, within this constraint, all connections carrying the column